The Hubble telescope has detected a long-sought population of comets dwelling at the icy fringe of the solar system. The observation, which is the astronomical equivalent to finding the proverbial needle-in-a-haystack, bolsters proof for a primordial comet reservoir just beyond Neptune. The circles pinpoint one of the candidate Kuiper belt objects. The dotted lines represent a possible orbit that this Kuiper belt comet is following.

Based on the Hubble observations, a team of astronomers estimate that the belt contains at least 200 million comets, which have remained essentially unchanged since the birth of the solar system 4.5 billion years ago.

NASA's Hubble Space Telescope has detected a long-sought population of comets dwelling at the icy fringe of the solar system. The observation, which is the astronomical equivalent to finding the proverbial needle-in-haystack, bolsters proof for a primordial comet reservoir just beyond Neptune, currently the farthest planet from the Sun.

Based on the Hubble observations, a team of astronomers consisting of Anita Cochran of the University of Texas, Austin, TX, Hal Levison and Alan Stern of Southwest Research Institute, San Antonio, TX branch office in Boulder, CO, and Martin Duncan of Queen's University, Ontario, Canada, estimate the belt contains at least 200 million comets, which have remained essentially unchanged since the birth of the solar system 4.5 billion years ago.

"For the first time, we have a direct handle on the population of comets in this outer region. The solar system just got a lot more interesting," said Cochran. "We now know where these short-period comets formed, and we now have a context for their role in the solar system's evolution."

The existence of a comet-belt encircling our solar system – like the rings which wrap around Saturn – was first hypothesized more than 40 years ago by astronomer Gerard Kuiper. The so-named Kuiper Belt remained theory and conjecture until 1992, when ground-based telescopes began detecting about 20 large icy objects ranging from 60 to 200 miles in diameter. The planet Pluto is considered by astronomers to be the largest member of the Kuiper Belt region. However, researchers had to wait for Hubble Space Telescope's high spatial resolution and sensitivity before they could search for an underlying population of much smaller bodies assumed to be present – just as there are more pebbles on the beach than boulders.

"This is a striking example of what Hubble can do well," said Cochran. "We can at last identify small comet-sized objects that are just a few miles across, about the size of New York's Manhattan Island. "Cochran discussed her team's findings at a 11:00 a.m. news conference June 14, at the 186th meeting of the American Astronomical Society in Pittsburgh, PA.

The team believes this apparently closes the mystery of the source of the short period comets, that orbit the Sun in less than 200 years, including such members as comet Encke, Giacobini-Zinner, and the infamous comet Shoemaker-Levy 9 that collided with the planet Jupiter in July, 1994. The comet-disk lies just beyond Neptune and might stretch 500 times farther from the Sun than Earth. This is 100 times closer to Earth than the hypothesized Oort cloud, commonly thought to be a vast repository of comets that were tossed out of the early solar system. Despite their close proximity, the Kuiper belt comets don't pose any greater threat of colliding with Earth than comets that come from much farther out, said experts.

The comet nuclei are the primordial building blocks that condensed out of the cloud of gas, dust and ices that collapsed to form the Sun. "Knowing where comets come from will help constrain models for the formation of the solar system and tells us something new about where we came from," Cochran emphasized.

"The Kuiper Belt is the best laboratory in the solar system for studying how planets formed," said Levison. "We believe we are seeing a region of the solar system where the accumulation of planets fizzled out."

The icy nuclei are too far away to have the characteristic shell (coma) and tail of gasses and dust that are a comet's trademarks, when it swings close enough to the Sun to warm up and sublimate. Detecting these bodies in their "deep-freeze" state, at the dim horizon of the solar system, pushed Hubble Space Telescope to its performance limits. "Imagine trying to see something the size of a mountain, draped in black velvet, located four billion miles away," said Stern.

The team used Hubble's Wide Field Planetary Camera 2 (WFPC 2) to observe a selected region of the sky in the constellation Taurus, that had few faint stars and galaxies that would confuse the search. The detection is based purely on a statistical approach, because the objects being discovered are so faint.

The team plans to continue searching for more objects. They have already collected more images with Hubble. These additional images allow them to better quantify the number and sizes of comets in the Kuiper belt. They also will apply for more Hubble observing time in the future to probe the structure of the Kuiper belt.

BACKGROUND INFORMATION: THE SEARCH FOR THE KUIPER BELT

In 1950, Dutch astronomer Jan Oort hypothesized that comets came from a vast shell of icy bodies about 50,000 times farther from the Sun than Earth is. A year later astronomer Gerard Kuiper suggested that some comet-like debris from the formation of the solar system should also be just beyond Neptune. In fact, he argued, it would be unusual not to find such a continuum of particles since this would imply the primordial solar system has a discrete "edge."

This notion was reinforced by the realization that there is a separate population of comets, called the Jupiter family, that behave strikingly different than those coming from the far reaches of the Oort cloud. Besides orbiting the Sun in less than 20 years (as opposed to 200 million years for an Oort member), the comets are unique because their orbits lie near the plane of the Earth's orbit around the Sun. In addition, all these comets go around the Sun in the same direction as the planets.

Kuiper's hypothesis was reinforced in the early 1980s when computer simulations of the solar system's formation predicted that a disk of debris should naturally form around the edge of the solar system. According to this scenario, planets would have agglomerated quickly in the inner region of the Sun's primordial circumstellar disk, and gravitationally swept up residual debris. However, beyond Neptune, the last of the gas giants, there should be a debris-field of icy objects that never coalesced to form planets.

The Kuiper belt remained theory until the 1992 detection of a 150-mile wide body, called 1992QB1 at the distance of the suspected belt. Several similar-sized objects were discovered quickly confirming the Kuiper belt was real. The planet Pluto, discovered in 1930, is considered the largest member of this Kuiper belt region. Also, Neptune's satellites, Triton and Nereid, and Saturn's satellite, Phoebe are in unusual orbits and may be captured Kuiper belt objects.

Observational Techniques

To isolate and subtract the effects of cosmic ray strikes on the WFPC 2's electronic detectors, which could mimic the faint signature of a comet, thirty-four images were taken of the same piece of sky. The cosmic ray hits change from picture to picture, but real objects remain constant. However, pinpointing comets was even trickier because they drift slowly along their orbit about the Sun. Although the orbital periods of these objects are 200 years or longer, the HST has sufficient spatial resolution to see them move in just a few minutes. This means the comets change position from picture to picture, just as cosmic ray strikes would. However, cosmic ray strikes are randomly placed events while the motions of the comets are well defined.

To distinguish between the comets and cosmic ray effects, the 34 images were then digitally shifted and stacked to the predicted offset to account for the expected drift rate of comets. It's like having a fixed camera on a tripod take a rapid series of snapshots of someone walking in front of the lens. The resulting snapshots could be stacked so that the person appeared stationary.

The researchers tested the reliability of this approach by shifting the stacked pictures in the opposite direction of the expected comets' motion. Ideally, no comets should have appeared, but random alignments added up to 24 anomalous detections.

When the team stack-shifted the pictures in the direction of the predicted comet motion, they came up with 53 objects. Assuming that 24 of these are, statistically, anomalous too, leaves a remainder of 29 objects considered "real."

The shift-stack technique was further tested by dividing the images into two groups and running an automated search algorithm to look for objects that showed up in the same position on sets of exposures.